See official C3 engine tools here: https://mega.nz/#F!FUoEGbIL!G37IIhwm2K8g08SnwhGq1A
Bitturn - Contributed by Pro4Never
Limited usefulness but this DOES work to export .c3 files so they can be viewed/edited in 3dsmax. Does not seem to work to convert back to .c3 so you'll still need the export tool above.
Dat Files - Contributed by high9
Pro4Never: “Many of the client files are encrypted with a basic hash. This includes things like itemtype.dat, monstertype.dat and magictype.dat. Before you can edit them you must decrypt them and once you are done your edits you have to re-encrypt them to see your edits in game. (note: old clients do NOT use encrypted files but rather binary file structures. Write your own converter or use a hex editor to edit these (HUGE pain without writing a custom tool).”
DBC Files - Contributed by Ultimation
Pro4Never: “Newer clients encrypt the .ini files so that they cannot be easily edited. You will need to decrypt them using a DBC tool and re-encrypt them when you are finished editing.”
DDS Plugin: Photoshop - Contributed by Pro4Never
Most modern versions of Photoshop already support DDS files but still useful to have the link in case you need it.
DDS Plugin: Gimp - Contributed by Pro4Never
Gimp is a free alternative to Photoshop.
NPC LookFace Extractor - Authored by Karim.Shadow
A command-line application for extracting all NPC LookFaces and generates an enumeration list formatted in C#. Requires access to the game client from its top directory.
WDF Archives - Contributed by Korvacs
By default it will not override existing files, so you don't need to worry about backing up existing modifications, you can also change the destination directory so that you can store your extracted files somewhere else, however the client obviously wont use them if they aren't in the right place.